(1) Field of the Invention
The present invention relates to a patch density measuring apparatus which measures the density of patches formed on a transfer medium and an image forming apparatus which has such a patch density measuring apparatus.
(2) Background Art
In order to avoid the density of an image formed by toner from being changed due to a secular change in apparatus and an environmental change, there has been conventionally known an electrostatographic image forming apparatus which has functions to form patches (test pattern), measure the patch density, and correct the density of the image based on the measurement result (for example, see Publication of Japanese Patent No. 3272768).
In a density measuring apparatus provided to the image forming apparatus, when the patch density is measured, the density corresponding to the dirt or scratch of the surface of a transfer medium (base) is measured by a sensor. The sensed voltage signal is converted into digital form by an A/D converter, and stored in a storage device. And then, patches are formed on the transfer medium. The patch density is measured by the sensor. And the analog-digital conversion is performed by the A/D converter. However, when the patch density is measured, the voltage signal stored in the digital form in the storage device upon measuring the surface of the transfer medium is converted into analog form, and is used as a reference voltage for deciding the dynamic range of the A/D convertor. This configuration allows the dynamic range of the A/D converter to be set within a minimal range, so that the accurate density measurement can be achieved.